Ontogenesis of structure-function relations between the neurons of the mesencephalic nucleus of the trigeminal nerve and jaw musculature as determined by jaw movements at different developmental stages is of great theoretical and practical interest. The fact that both nervous and endocrine systems develop side by side in an orderly sequence suggest that their relationships may be laid during ontogeny. During neural ontogeny there is evidence of the action of a specific hormone on a particular cell type at a definite time during development. Disturbances in jaw movements and related reflexes are presumably behavioral consequences of interference with cell differentiation at a critical period of development. It is our belief, that thyroid hormone is responsible to a considerable extent, for the growth and maintenance of neuronal elements involved in this reflex pathway during ontogeny. The proposed study using birds as experimental material seeks to clarify the precise origin of the cells of the mesencephalic nucleus by the method of heterotransplantation of cells from quail as biological markers, and an ultrastructural characterization of features such as cellular morphology, relative number and distribution of cells within the nucleus correlated with behavioral studies (jaw movements or beak clapping) qualitatively and quantitatively under (a) normal and (b) experimental conditions in which an antithyroid drug is injected intravenously during embryonic stages such that the thyroid in the embryo is inhibited. By this method we intend to establish: (1) the precise brain region from where the precursor cells of the mesencephalic nucleus originate, when and how they come to be localized, (2) the effects of hypothyroidism on jaw movements; (3) ultrastructural characterization of the neurons of the mesencephalic nucleus under normal conditions; (4) characterization at the EM level of changes if any, in the nurons of the mesencephalic neclus in the hypothyroid state.